1. Field of the Invention
The present invention relates to phase comparing method and apparatus, and more specifically to phase comparing method and apparatus for use in a servo control system of a mechanism including a rotating part, such as a spindle motor, for maintaining the speed and the phase of the rotation of the rotating part constant.
2. Description of Background Information
A phase comparator is generally used in control systems of mechanisms having a rotating part for generating a drive speed control signal. In such control systems, the control signal is generated in response to a phase difference between a frequency signal which is in synchronism with the rotation of the rotating part, such as a playback sync signal recovered from a recording disc, and a reference signal generated by a source of reference signal.
An example of conventional phase comparing circuitry being used in a disc player for controlling a disc drive speed is shown in FIG. 1.
As shown, a reference oscillation signal from a reference signal generator 1' is supplied to an MMV (monostable multivibrator) 2' where the input signal is converted to a pulse train signal (A') consisting of pulses having a constant pulse width. A capacitor 3' having a terminal connected to a current source 5' is provided and charge and discharge of this capacitor 3' is controlled by opening and closing of a switch 4' which is connected to the terminal of the capacitor 3'. The opening and closing of the switch 4' is controlled by means of the pulse train signal supplied from the MMV 2'. Specifically, the capacitor 3' is charged when the switch 4' is turned off (opened) so that a voltage (B') for sampling, i.e. a ramp-form, or sawtooth, signal appears at the terminal of the capacitor 3'. The ramp-form signal is supplied to a sample switch 7' through a buffer amplifier 6' which has a voltage shifting function for setting a center voltage of the ramp-form signal at zero level.
A sampled output signal produced from the sample switch 7' is held by means of a holding capacitor 8' and a signal (D') held by the holding capacitor 8' is used as the phase error signal.
On the other hand, an RF signal whose phase error is to be detected, such as a playback video signal, is demodulated at a demodulator 9' and in turn supplied to a sync (synchronization) detection circuit 10' in which a playback sync signal is extracted from a demodulation signal from the demodulator 9'. A sample pulse generator 11' is connected to the sync detection circuit 10' so as to produce a sampling pulse train signal (C') made up of a plurality of sampling pulses, in synchronism with the playback sync signal.
With reference to waveform diagrams of FIGS. 2A throgh 2D which respectively show waveforms of the signals (A') through (D'), the operation of the phase comparator circuitry shown in FIG. 1 will be explained.
When a phase change of the playback signal occurs, the phase of the sampling pulse signal changes accordingly. Therefore, sampling timing of the ramp-form signal which is produced at the terminal of the capacitor 3' and supplied to the sampling switch 7' is varied in response to the phase change in the playback sync signal. The level of the phase error signal developed at the terminal of the holding capacitor 8' is varied in this way.
With this type of arrangement, a proper phase error signal can be produced so far as the frequency of the playback sync signal is near to the frequency of the reference signal. However, if the frequency difference between the playback sync signal and the reference signal becomes large, the generation of proper phase error signal becomes no more possible because of the reason which is described in detail below.
When, for example, the frequency of the playback signal drops significantly, the interval of the sampling pulses increases accordingly. Therefore, as shown in FIG. 3, the level of the error signal (D') increases, from a zero level, towards a peak level appearing at a top portion of each slope of the ramp-form signal (B") which appears at an output terminal of the buffer amplifier 6 and shown in this figure for the explanation purpose. At the peak of the rise, the phase error signal reverses the direction and decreases immediately towards a negative side as a result of the sampling of the ramp-form signal at its portions of the lowest level. Then, the error signal increases with time towards the peak level and this type of change occurs repeatedly. In this way, the phase error signal oscillates between the peak level and the lowest level of the ramp-form signal like a saw tooth wave, as typically illustrated at the line (D') in FIG. 3. The frequency of the above explained oscillation of the phase error signal increases as the frequency difference between the playback signal and the reference signal becomes large.
If the phase error signal under the oscillating state as explained above is utilized for controlling the drive of the rotating part such as a spindle motor for supporting a disc in a disc playing system, it becomes very difficult to perform a proper control operation, especially, lock-in or synchronization of a servo system which is generally provided in the disc playing system becomes difficult.
Because of the reason stated above, it is general to provide, in addition to the phase error detection circuit 20 such as the circuit shown in FIG. 1, a frequency difference detector 30 and an adder circuit 31 for adding the phase error signal from the phase error detection circuit and an output signal of the frequency difference detector, as illustrated in FIG. 4.
By adding the frequency difference detector 30, it becomes possible to control the rotary mechanism within a speed range permitting a pull-in of the servo system by means of the output signal of the frequency difference detector, because an average of the level of the output signal of the phase error detection circuit under the oscillating state is substantially equal to zero. When a difference between the speed of the rotary mechanism and a target speed has decreased sufficiently, the servo control of the driving by means of the output signal of the phase error detection circuit is started.
However, in the case of this type of measure, it is disadvantageous that two detection systems which are independent from each other are required and an interaction between off-set levels of the detection systems is difficult to avoid, and complicated adjusting operations are required for ensuring an accurate and smooth operation of the servo system.